Not applicable.
Not applicable.
Not applicable.
1. Technical Field
The invention relates to the combination of two different methods that are used in electron beam lithography, namely electron beam writing and electron beam projection lithography.
2. Background Art
In electron beam writing, the substrate is sequentially exposed by means of a focused electron beam, wherein the beam either scans in the form of lines over the whole specimen and the desired structure is written on the object by corresponding stopping-out of the beam, or, as in the vector scan method, the focused electron beam is guided only over the regions to be exposed. Electron beam writing is distinguished by high flexibility, since the circuit geometries are stored in the computer and can be optionally varied. Furthermore, very high resolutions can be attained by electron beam writing, since electron foci with diameters smaller than 100 nm can already be attained with simple electron-optical imaging systems. However, it is disadvantageous that the process is very time-consuming, due to the sequential, pointwise writing. Electron beam writing is therefore at present mainly used for the production of the masks required in projection lithography.
Electron beam writers are based, as regards equipment technology, on scanning electron microscopes, which are as a rule considerably simpler than transmission electron microscopes. In addition to the usual components for a scanning electron microscope, all that is further required is a so-called beam-blanker, by means of which the electron beam can be deflected onto a diaphragm in order to switch the electron beam xe2x80x9coffxe2x80x9d at the places which are not to be exposed.
In electron beam projection lithography, analogously to optical lithography, a larger portion of a mask is illuminated simultaneously and is imaged on a reduced scale on a wafer by means of a projection optics. Since a whole field is imaged simultaneously in electron beam projection lithography, the attainable throughputs can be markedly higher in comparison with electron beam writers. However, due to the lens aberrations of uncorrected electron-optical systems, only subfields of the mask, of size about 1 mmxc3x971 mm, can be simultaneously imaged on a reduced scale on the wafer. For the exposure of a whole circuit, these subfields have to be placed against each other by an electron-optical or mechanical displacement, or by a combination of the two displacement methods.
A corresponding electron beam projection lithography system is known, for example, from U.S. Pat. No. 3,876,883. It is already described there that for adjusting the mask and wafer relative to each other, the excitation of the condenser ahead of the mask can be varied such that the electron beam is focused on the mask. The subsequent projection system then images onto the wafer the electron focus arising in the mask plane.
Further similar electron beam projection lithography systems are described, for example, in U.S. Pat. No. 4,140,913 and in European Patent Document 0 367 496.
A disadvantage of electron beam projection lithography systems is that a corresponding mask is necessary for each structure to be exposed. The preparation of customer-specific circuits in small numbers is not economic, because of the high costs associated with mask production.
A known intermediate form between electron beam writing and electron beam projection lithography is writing with a shaped electron beam. Instead of a focused electron beam, the profile of the electron beam is shaped using a diaphragm, and the diaphragm is projected onto the substrate to be exposed. The diaphragm apertures have standard geometric shapes, the overall pattern to be produced on the substrate then being combined from these geometric standard shapes. This variant thus manages without specific masks, but is only slightly faster than writing with a focused electron beam, and is markedly slower than electron beam projection lithography.
The object of the present invention is to provide a method and an electron beam projection lithography system with which customer-specific circuits can be produced very economically, even in small numbers.
This object is attained by an electron beam projection lithography system comprising an electron source providing an electron beam, a condenser system, a mask plane behind the condenser system as seen in the direction of the electron beam, a substrate plane, a projective system that follows the mask plane as seen in the direction of the electron beam and is excitable such that the mask plane is imaged on a reduced scale in the substrate plane, a control system, and a projector deflection system in or before the projective system as seen in the direction of the electron beam, wherein the control system is arranged to change over condenser excitation or deflection elements such that a small focused or shaped beam profile is produced is the mask plane and wherein the projector system is driven such that a focused or shaped electron beam with a small beam profile is movable in the substrate plane along stored or computed paths. The object is attained by a method of electron beam lithography comprising in a first step electron-optically imaging a mask by a projective system on a substrate to be exposed arranged in a substrate plane, and in a second step guiding a focused electron beam or an electron beam with a shaped profile over the substrate by focusing the electron beam in a plane of the mask or shaping a beam profile of the electron beam before the plane of the mask and deflecting the focused electron beam or the electron beam with a shaped profile by a deflection system.
The present invention is based on a combination of electron beam projection lithography with electron beam writing in a single apparatus. In the method according to the invention, firstly a mask is electron-optically imaged in a first step onto the substrate to be exposed by means of a projective system. For this purpose, the mask has the coarser structures to be produced and/or universally necessary structures. In a second step, by focusing the electron beam in the mask plane or by forming the electron beam ahead of the mask plane by means of a diaphragm, further imaging onto the substrate to be exposed of the focus arising in the mask plane, or of the image of the beam shaping diaphragm arranged ahead of the mask plane, and targeted deflection of the electron focus or of the shaped electron beam in the substrate plane by a deflecting system, the fine structure not present in the mask but nevertheless necessary, and/or the conductor paths and other structures not present in the mask but nevertheless corresponding to the customer""s requirements, are written onto the substrate.
Electron beam writing takes place, according to a first embodiment of the invention, by means of an electron beam that is focused in the substrate plane. In a second embodiment, the electron beam writing takes place with an electron beam shaped by a diaphragm, the diaphragm then having regions that have standard shapes and are transmissive for electrons.
Both steps of the combination according to the invention can of course be carried out one after another with multiple iterations, for the exposure of larger fields on the substrate. Since both steps are carried out in multiple succession with the same apparatus, no readjustment of the substrate relative to the optical axis of the apparatus is required between the two steps.
An electron beam projection lithography system according to the invention has an electron source, a preferably multi-stage condenser, a mask plane provided following the condenser, and a projective system following the mask plane. The projective system can be excited such that the mask is imaged on the substrate on a reduced scale. By means of a control system, the condenser excitation can be changed over, or can be deflected onto a further diaphragm, such that the electron beam alternatively either uniformly illuminates the mask plane over a larger field, or is focused in the mask plane, or has a small, shaped beam cross section in the mask plane. A deflection system is furthermore provided in or before the projective system and can be driven such that a focused or shaped electron beam can be moved over the substrate along stored or computed paths.
Differing from the lithography system known from U.S. Pat. No. 3,876,883, in the lithography system according to the invention, with the electron beam focused in the mask plane, the electron beam is scanned with the projective scanner. For this purpose, the projective scanner is coupled to a pattern generator that produces the structures to be written. The condenser deflecting system is constantly driven in this mode of writing so that the electron beam can pass through a hole in the mask.
In an embodiment of the invention with a multi-stage condenser, a condenser aperture diaphragm is provided which is arranged, seen in the beam direction, ahead of the mask plane. The plane in which this condenser aperture diaphragm is arranged is then to coincide with that plane in which an image of the electron source arises in the projection mode and thus with uniform illumination of the mask plane. This condenser aperture diaphragm is then without importance in the projection mode, and serves in the writing mode simultaneously for the definition of the illumination aperture and also as a dark scanning diaphragm through the deflection, by means of a condenser deflection system, of the electron beam onto the diaphragm at places which are not to be exposed.
A further diaphragm, a field diaphragm in the projection mode, is furthermore preferably provided between the condenser lenses. This field diaphragm is then arranged in a plane that corresponds to the source-side object plane of the last condenser lens and consequently is sharply imaged in the mask plane by the last condenser lens.
It should be mentioned at this point that the changeover between the writing mode and the projection mode takes place by a change of the excitation of the first, source-side condenser lenses, or by targeted deflection of the electron beam onto a diaphragm in the condenser and that excitation of the last condenser lens preceding the mask plane, is constant in both modes, so that the focal planes and the entry and exit image planes of the last condenser lens are fixed in both modes.
In a further embodiment, the last condenser lens and the first projective lens are formed by a single, so-called condenser-objective single field lens, with the mask plane situated in the gap center of the condenser-objective single field lens. By this means, the known small axial aberration coefficients, in particular energy dependent aberration coefficients, known for condenser-objective single field lenses can be realized.
Since in the system according to the invention the projective system is also not changed, as regards its excitation, on changing over between writing mode and projection mode, the last lens of the projective system can furthermore also be constituted as a condenser-objective single field lens, only the effect of the entry-side field of this lens being used for the imaging of the electron beam. Although this may appear at first sight to be over-dimensioned, since only the entry-side partial field of this second condenser-objective lens is used, some advantages nevertheless accrue. Since the overall system, in spite of its high flexibility, needs only two condenser lenses and two condenser-objective single field lenses, and both condenser-objective single field lenses can have a substantially identical construction and are to differ only as regards a linear scaling factor which corresponds to the imaging scale between the mask plane and the substrate plane, an advantage results in the first place as regards production technology. With a geometrically similar construction of both condenser-objective single field lenses, there furthermore results a corresponding geometrical similarity of the magnetic fields of both projection lenses, so that the off-axis aberrations e.g. the isotropic and anisotropic distortion, of both projective lenses can be mutually compensated. For this aberration compensation, the two condenser-objective single field lenses are to be operated with mutually inverse polarization of the focusing magnetic fields.
A further advantage of a second condenser-objective single field lens is the good detectability of secondary electrons emergent from the substrate to be exposed, since the substrate is arranged in the focusing magnetic field of the condenser-objective single field lens, and the secondary electrons are collected by this magnetic field in a known manner.
In the method according to the invention and in connection with the system according to the invention, a mask is preferably used which has subfields separated from each other by webs. Successive different subfields can then be uniformly illuminated by the deflection of the electron beam by means of the deflecting system provided in the condenser, and can be successively projected, spatially adjoining each other, by suitable return deflection of the electron beam by the deflection system in the projective system.
The mask furthermore preferably has holes in the webs, having a diameter greater than the diameter of the electron beam focused by the condenser. The electron beam in writing mode is then deflected onto these holes, so that the electron beam can pass through the mask unhindered. A suitable arrangement of the holes furthermore permits at least a coarse adjustment of the mask to the electron beam.
Additionally, for the sequential exposure of fine structures or customer-specific structures, the electron beam focused in the mask planexe2x80x94or shaped ahead of the mask planexe2x80x94can of course also serve for the adjustment of the subfields of the mask relative to the substrate to be exposed, as is described in the above cited U.S. Pat. No. 3,876,883. Furthermore, the electron beam focused in the mask planexe2x80x94or shaped ahead of the mask planexe2x80x94can also be used for post-exposure of erroneous mask structures or for the elimination of holes in the mask by electron beam stimulated metal deposition, and can thus be used for mask repair. The system according to the invention is thus distinguished as a whole by very flexible possibilities of use.